1. Field of the Invention
This invention relates to a roll for a tape guide and a manufacturing method therefor. More particularly, it relates to a roll for a tape guide provided on a tape cassette for guiding it and a manufacturing method therefor.
2. Description of Related Art
A tape cassette or an apparatus for recording or reproducing information signals on or from the tape cassette is provided with a tape guide roll for guiding the tape.
As this tape guide roll, there are known a substantially cylindrically-shaped tape guide roll a, manufactured by injection molding from a molten resin (see FIG. 1), and tape guide rolls e,f formed as-one from an inner cylinder b, an outer cylinder c and a plurality of connecting portions d interconnecting the inner cylinder b and the outer cylinder c with a radial spacing in-between (see FIGS. 2 and 3).
If, in general, the tape guide roll is to be formed unitarily by injection molding, the molten resin charged into the cavity is cooled and solidified to produce a sink. If this sink is excessive, the outer peripheral surface cannot be formed as concentric cylindrical surface and hence certain precision of the molded product (tape guide roll) cannot be maintained, thus occasionally affecting tape characteristics to produce hindrance in tape running.
Thus, in the aforementioned tape guide rolls a, e or f, an injection-molded product has its outer periphery machined to improve the precision in shape in order to form the outer peripheral surface as an optimum concentric cylindrical surface.
However, if the machining is used to improve the precision in profile, there is raised a problem that machining is labor-consuming to raise the production cost. Moreover, machining precision is also a requirement.
On the other hand, if the tape guide roll is to be manufactured by injection molding, the injection pressure in the cavity differs appreciably depending on the position of a gate provided in a molding device for injection molding. Moreover, unneeded load is applied to the molded product at the time of ejection, depending on the ejecting position by an ejection pin, thus occasionally worsening the precision in shape of the molded product.
It is therefore an object of the present invention to provide a tape guide roll and a manufacturing method therefor, in which it is possible to maintain an optimum tape running state.
In one aspect, the present invention provides a tape guide roll produced by injecting a molding material into a pre-set cavity and formed unitarily from an inner cylindrical portion, an outer cylindrical portion coaxial as the inner cylindrical portion, and a plurality of connecting portions arranged in a spaced apart relationship from one another along the circumferential direction for interconnecting the inner and outer cylindrical portions, wherein the length in the axial direction of the inner cylindrical portion is set so as to be longer than a tape width, with the length in the axial direction of the outer cylindrical portion being longer than the length in the axial direction of the inner cylindrical portion.
Thus, optimum tape running is assured even in a tape guide roll which is produced without machining.
In a preferred embodiment of the present invention, the length in the axial direction of the inner cylindrical portion and the connection portion is not smaller than 90% of the length in the axial direction of the outer cylindrical portion. By so doing, there is scarcely any difference in the force of contraction in the respective portions along the axial direction of the tape guide roll to eliminate inconveniences such as gate breakage, whilst the outer periphery of the outer cylindrical portion can be formed as a concentric cylindrical surface of a high accuracy.
In another present embodiment of the present invention, the thickness of the inner cylindrical portion and the thickness of the outer cylindrical portion are set so as to be not larger than 0.5 mm and the thickness in the circumferential direction at least of both end portions of the connecting portions connecting to the inner and outer cylindrical portions is set so as to be not larger than 0.5 mm. As a result, the tape guide roll can be provided in which the force of contraction during cooling and solidification of the molten resin is suppressed and the sink is minimized to assure optimum accuracy in the respective portions.
In another aspect, the present invention provides a manufacturing method for a tape guide roll in which a molding material is injected into a pre-set cavity to unitarily form an inner cylindrical portion, an outer cylindrical portion coaxial with the inner cylindrical portion, and a plurality of connecting portions arranged in a spaced apart relationship from one another along the circumferential direction for interconnecting the inner and outer cylindrical portions, wherein the method includes a step of injecting the molding material into the cavity from a gate arranged at a position facing an end face of a portion which is to form the connecting portion, thrusting the opposite end face to the one end face of the connecting portion by an ejecting pin and simultaneously thrusting the outer cylindrical portion to project the tape guide roll a pre-set amount and ejecting the tape guide roll by a stripper plate after projecting the tape guide roll the pre-set amount.
In this manufacturing method, the injection pressure in the cavity is improved in uniformity, such that the molten resin is injected substantially uniformly into respective parts of the cavity, at the same time the mold release resistance at the time of ejection is diminished to maintain optimum molding accuracy of the tape guide roll.
Since the ejection is initially by the ejection pin and by the stripper plate, and subsequently by solely the stripper plate, there is left no mark of ejection by the ejection pin. In addition, no unneeded load tends to be partially applied to the molded product during its ejection.